Lithium secondary batteries, nickel hydride batteries, and other secondary batteries have in recent years entered into use as vehicle-mounted power sources and as power sources for personal computers and portable and mobile electronic devices. In particular, the importance of lithium secondary batteries, which are light and provide a high energy density, as high-output vehicular power sources is growing. A typical configuration for this type of lithium secondary battery is provided with an electrode assembly having a structure in which sheet-shaped electrodes are wound into a spiral shape (wound electrode assembly). To form this wound electrode assembly, for example, a positive electrode sheet having a structure in which a positive electrode active material layer containing a positive electrode active material is supported on both sides of a positive electrode current collector and a negative electrode sheet having a structure in which a negative electrode active material layer containing a negative electrode active material is supported on both sides of a negative electrode current collector are wound into a spiral configuration with a separator sheet interposed therebetween.
A porous resin sheet comprising, e.g., polyethylene (PE) and/or polypropylene (PP), has been used as the separator sheet. This separator sheet, being porous, undergoes heat shrinkage when the temperature becomes elevated. This is used to generate a shutdown function. However, when a large heat shrinkage occurs, localized short circuiting is produced by, for example, film rupture, and additional short circuiting can spread from that location. The formation of a filler layer (porous heat-resistant layer) on the surface of the separator sheet has thus been proposed in order to prevent heat shrinkage by the separator sheet (for example, Patent Literature 1).